In the EDM field, it is known to translate the electrode tool and the electrode workpiece relative to each other according to a cyclical motion, or orbiting motion, along a path situated on a surface of revolution of a predetermined shape, with the result that the machining gap between the electrodes is decreased over a portion of the machined surface and the machining zone is caused to be cyclically displaced. It is also known to vary as a function of the machining gap width, at least during one orbital cycle, the speed at which the cyclical orbital motion is effected such as to remove, from all the machining surface of the workpiece, the same quantity of material per unit of surface, as disclosed in U.S. Pat. No. 4,057,703.
According to those known processes, however, it is not possible to simultaneously control constantly the machining conditions, that is the machining gap width, while maintaining the accuracy of the workpiece shape. This is due to the fact that the known processes consist in limiting to a predetermined path the orbiting trajectory and in varying the speed of relative translation of the electrodes as a function of the variations of a quantitative value representing the machining gap width. A simultaneous control of the machining gap width and of the speed of relative displacement of the electrodes can be accomplished only by providing a stored program of the speed once the motion path has been predetermined.
The present invention, by contrast, permits to control the shaping of the workpiece surface relative to the tool shape while simultaneously controlling the machining gap width. The control of the orbiting speed is effected as a function of another magnitude which varies as a function of the amount of mismatch between the shape of the surface machined on the workpiece and that of the electrode tool. This magnitude is equal to the instantaneous amplitude of the translation motion.
The present invention has therefore for principal object to improve the machining efficiency of an EDM machine and to provide a continuous positive control of the workpiece shape as machining progresses, such as to uniformly enlarge the shape of the workpiece.